This relates generally to electronic devices, and more particularly, to electronic devices with displays.
Electronic devices often include displays. For example, cellular telephones and portable computers often include displays for presenting information to a user.
Liquid crystal displays contain a layer of liquid crystal material. Display pixels in a liquid crystal display contain thin-film transistors and electrodes for applying electric fields to the liquid crystal material. The strength of the electric field in a display pixel controls the polarization state of the liquid crystal material and thereby adjusts the brightness of the display pixel.
Substrate layers such as color filter layers and thin-film transistor layers are used in liquid crystal displays. The thin-film transistor layer contains an array of the thin-film transistors that are used in controlling electric fields in the liquid crystal layer. The color filter layer contains an array of color filter elements such as red, blue, and green elements. The color filter layer provides the display with the ability to display color images.
In an assembled display, the layer of liquid crystal material is sandwiched between the thin-film transistor layer and the color filter layer. Polyimide passivation layers cover the inner surface of the color filter layer and the upper surface of the thin-film transistor layer. An array of column spacers is formed on the inner surface of the color filter layer to maintain a desired gap between the color filter layer and the thin-film transistor layer. Column spacers are typically formed from hard organic materials such as photoresist.
There are tradeoffs involved in designing an appropriate column spacer arrangement for a display. If the column spacers do not provide sufficient support for a display, the display may be susceptible to an undesirable visual effect called pooling mura. On the other hand, column spacer designs that cause a display to become overly rigid will make the display prone to stress-induced birefringence when deformed, leading to undesired light leakage effects.
In order to provide sufficient support for a display, care must be taken to ensure that the column spacers are deposited on a flat surface. In a typical configuration, an array of red, green, and blue color filter elements are deposited on an inner surface of a color filter substrate. An array of column spacers is then formed over the array of color filter elements. Because the color filter elements have varying heights with respect to the inner surface of the color filter substrate, a transparent planarization layer is formed over the color filter array prior to forming the column spacer array.
The varying step height between red, green, and blue color filter elements requires a thick transparent overcoat layer in order to form a planar surface over the color filter array. This type of thick overcoat layer can cause display colors to appear washed out to a user.
It would therefore be desirable to be able to provide a display with an improved column spacer configuration.